Structures forming integrated circuits and other nanotechnology have dimensions on the nanometer scale. One method of observing the structures for purposes such as process development, process control, and defect analysis is to expose a portion of the structure using a focused ion beam (FIB) system and observing the exposed structure using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). When the ion beam mills material to expose a structure for observation, the ion beam can distort the structure and create artifacts that interfere with the observation.
A high aspect ratio (HAR) structure is a structure having a dimension, such as height, that is much greater than another dimension, such as its width. For example, a hole between layers in an integrated circuit may have a height that is several times greater than its width. For example, a feature having a height more than 3 times its width.
In analyzing high aspect ratio structures, especially unfilled contacts or vias, for the 3D structures in integrated circuits, such as 3D NAND circuits used in flash memory, the conventional ion beam sample preparation process causes artifacts, such as structure distortion, and the ion beam curtain effect.
The ion beam curtain effect or curtaining occurs when material is removed at different milling rates. This can happen when milling a feature comprised of materials that are removed at different rates by the same beam. This can also happen when milling a surface that has an irregular shape. For example, the feature of interest can be a through-silicon vias (TSV). Cross-sectioning TSVs is a common practice in semiconductor labs to characterize voids and surface interfaces. Due to the depth of TSVs (typically 50-300 nm), milling a cross section of a TSV with an ion beam can result in substantial curtaining.
Because of the damage and artifacts caused by the ion beam milling to expose the features, the images do not faithfully show the results of the fabrication process and interfere with measurements and with an assessment of the fabrication process since the image and measurements show the results of the sample preparation and not the manufacturing process. It also makes performing high aspect ratio vertical structure analysis difficult.
Producing curtain-less TEM samples of 3D NAND and other IC structures w/recurrent high aspect ratio holes such as vias or contacts is currently difficult or non-achievable. It has been difficult or impossible to retain shape integrity of high aspect ratio holes or trenches when milling or preparing with a FIB and/or imaging with a SEM. When there are unfilled holes on a sample there are high differentials in the milling rates between the material and areas adjacent to the open area (hole). The large difference in milling rates results in curtaining or water fall effects that distort the shape of the hole.
The FIB produces artifacts on open structures. Etched holes or trenches when processed for cross sections to TEM prep with a FIB are prone to severe curtaining artifacts. Making interpretation of the cross section or difficult or impossible. High aspect ratio holes or trenches with complex material stacks are difficult measure with other methods (scatterometry, CD-SEM, etc.) Plan view or glancing angle material removal allows access to various depths for measurement. However, such methods do not provide a view from an electron beam that is normal along the entire length of a high aspect ratio hole or via.
In the prior art, curtaining effects are mitigated by placing protective depositions across the top surface of the sample or by doing the highest offset angle mill possible given the sample geometry, even to the point of inverting the sample. The ability to re-orient a work piece such as a semiconductor wafer in a vacuum chamber of a FIB is typically limited. Gaining milling and viewing capabilities from multiple planes of a sample also presents a number of problems. Prior art techniques for manipulating a sample in a charged particle beam system are very limited, typically only allowing one or two planes of viewing. Current methods of chunking and welding can only provide limited information due to system hardware constraints. Lengthy time periods are required to make multiple welds and multiple sample manipulations. Manually loading and unloading the sample from system, flipping the sample, or placing the sample in a different holder can be required, further increasing processing time. Other methods include slicing through a region of interest in one plane and then reconstructing data to get information from other angles. This method is time consuming, however, requires forming a number of images after multiple mills and then reconstructing the data to form an image in a different plane.
What is needed is a way to expose regions of interest for examination and/or measurement and produce an accurate image that reflects the region of interest without damaging the region or creating artifacts in the exposed surface. What is also needed is an improvement over prior art milling and viewing capabilities from multiple planes of a sample.